(1) Field of the Invention
The present invention relates to a device and a method for detecting a pilot signal, and more particularly to a circuit and a method for detecting a pilot signal for use in a two-carrier sound multiplexing system which is one of sound multiplexing systems for television signals.
(2) Description of the Related Art
The television sound multiplexing system is a system of broadcasting in which a plurality of sound channels are provided for television broadcasting of, for example, bilingual dual sound programs and stereophonic sound programs. One of such systems, a two-carrier sound multiplexing system, which is described in CCIR Document, Report 795-3, 1978-1990, pages 205-221, is a television system used in Europe such as in a PAL television system.
The two-carrier sound multiplexing system is briefly explained with reference to FIG. 1 which diagrammatically show frequency spectrum thereof. The sound signal is transmitted by frequency modulation of two-carriers, that is, an inherent sound carrier of the television system being considered and an additional sound carrier whose frequency is higher than that of the inherent sound carrier. First and second sound IF (intermediate frequency) signals are obtained respectively from the corresponding two sound carriers that are received. The first and second sound IF signals are FM-detected, and a first and a second sound signal respectively corresponding thereto are outputted. In these signals, the first sound signal includes an L+R signal or a main sound signal and the second sound signal includes a 2R signal or a sub-sound signal respectively dependent on whether the type or mode of the sound multiplexing signal is a stereo type or a dual sound type. A pilot signal for discriminating the sound multiplexing signals is also included in the second sound signal. This pilot signal is one in which a signal whose frequency is 3.5 times (hereinafter referred to as "3.5f.sub.H ") that of a horizontal synchronization signal f.sub.H of a video signal is used as a carrier, and which is AM-modulated at modulation degrees respectively of 50% at a frequency of f.sub.H /133 (about 117.5 Hz) in the case of the stereo discrimination signal for stereo broadcasting and at a frequency of f.sub.H /57 (about 274.1 Hz) in the case of the dual sound discrimination signal for dual sound broadcasting. The main object of the pilot signal is to detect frequencies respectively of the AM-modulated stereo and dual sound signals and, by doing so, to determine the multiplex sound mode of the broadcast in progress.
FIG. 2 shows, in a block diagram, the most basic first conventional pilot signal detection circuit. This pilot signal detection circuit consists of a band-pass filter (BPF) 1 having a center frequency of 3.5f.sub.H, which receives a second sound signal A and extracts a pilot signal P; an AM detector 21 which AM-detects the pilot signal P and extracts a modulated signal M; a BPF 22 having a center frequency of 117.5 Hz for a stereo discrimination signal S and a BPF 23 with a center frequency of 274.1 Hz for a dual sound discrimination signal D each of which allows the passage of the modulated signal M; and an amplitude assessment circuit 24 which receives a supply of signals S and D, assesses the amplitude thereof and judges the mode of the broadcasting sound signal being received.
The circuit described above operates as follows: The second sound signal A is supplied to the BPF 1, and the BPF 1 allows the passage of the signal in the vicinity of 3.5f.sub.H and extracts only the pilot signal P. This pilot signal P is detected by the AM detector 21 and is supplied as modulated signals to the BPF 22 and the BPF 23. When the reception is of a stereo broadcasting, since the modulated signal M is 117.5 Hz, the signal passes through the BPF 22 of 117.5 Hz and a corresponding stereo discrimination signal S is outputted. Also, when the reception is of a dual sound broadcasting, since the modulated signal M is 274.1 Hz, the signal passes through the BPF 23 and a corresponding dual discrimination signal D is outputted. The amplitude assessment circuit 24 assesses amplitudes of the signals S and D and judges the mode of the broadcast being received by the discrimination signal whose amplitude is larger between them, and appropriately switches the modes of the sound signals that are outputted correspondingly to results of the judgment.
In the first conventional pilot signal detection circuit, the detection sensitivities of the stereo and dual sound discrimination signals S and D are largely dependent on the selectivity characteristics of each of the BPF 22 and the BPF 23 after the AM detection. For enhancing the selectivity characteristics, it is necessary that Q of these BPFs be maintained high. This means that the tolerance of a center frequency deviation is narrower and this is a reason that makes the integration of circuits difficult during the fabrication thereof.
FIG. 3 shows, in a block diagram, a second conventional pilot signal detection circuit which is intended to overcome the above mentioned problem and which is disclosed in Japanese Patent Application Kokai Publication No. Hei 2-105784. This detection circuit consists of a BPF 1 that extracts the same pilot signal P as in the first conventional example; a plurality of multipliers (MULTs) 6, 7, 8 and 9 that respectively multiply the pilot signal P with each of the reference signals g.sub.1(t), g.sub.2(t), g.sub.3(t) and g.sub.4(t) and respectively produce signals P.sub.1, P.sub.2, P.sub.3 and P.sub.4 ; and adder (ADD) 10 which adds the signal P.sub.1 and P.sub.2 and produces a signal U.sub.1 ; an adder 11 which adds the signals P.sub.3 and P.sub.4 and produces a signal U.sub.2 ; a pair of low-pass filters (LPFs) 12 and 13 that remove high harmonics of each of the signals U.sub.1 and U.sub.2 and respectively produce signals V.sub.1 and V.sub.2 ; an amplitude assessment circuit 14 that receives a supply of the signals V.sub.1 and V.sub.2, makes amplitude assessment and judges the mode of the broadcasting sound signal being received; and a reference signal generation circuit 16 that produces the above reference signals g.sub.1(t), g.sub.2(t), g.sub.3(t) and g.sub.4(t).
The operation of the second conventional pilot signal detection circuit described above is now explained. The pilot signal P extracted by the BPF 1 is supplied to one of input terminals of each of the multipliers 6-9. The reference signals g.sub.1(t) -g.sub.4(t) are supplied to the other of the input terminals of each of the multipliers 6-9. Each of the multipliers 6-9 is generally a combiner and is constituted by a non-linear element such as a diode and a parallel modulator or the like. Each of the reference signals g.sub.1(t) -g.sub.4(t) is a combined signal wherein a signal whose frequency is 3.5f.sub.H, which is the same as that of the carrier of the pilot signal P, is combined with a signal whose frequency is f.sub.H /18, which is the same as that of the stereo discrimination signal, or a signal whose frequency is f.sub.H /57, which is same as that of the dual sound discrimination signal. Since the difference between the stereo and the dual sound discrimination signals resides only in the frequencies, the following explanation is made only for the stereo discrimination signal for convenience.
The reference signals g.sub.1(t) -g.sub.4(t) may be expressed using the equations as given hereunder.
g.sub.1(t) =cos.omega..sub.p t.multidot.cos.omega..sub.s t PA1 g.sub.2(t) =sin.omega..sub.p t.multidot.sin.omega..sub.s t PA1 g.sub.3(t) =sin.omega..sub.p t.multidot.cos.omega..sub.s t PA1 g.sub.4(t) =cos.omega..sub.p t.multidot.sin.omega..sub.s t PA1 .omega..sub.p t: pilot signal carrier frequency (3.5f.sub.H). PA1 .omega..sub.s t: stereo discrimination modulation frequency (f.sub.H /18=177.5 Hz). PA1 a first and a second multiplying circuit which multiply the pilot signal respectively with a first reference signal that is a first circular function of the third frequency and a second reference signal that is a circular function of the third frequency, and produce respectively a first and a second multiplied signal; and PA1 a first and a second low-pass filter which carry out a predetermined low-pass filtering in response to a supply of the first and the second multiplied signal, and produce respectively a first and a second filtered signal which are supplied to the second multiplying and filtering stage. PA1 multiplying the pilot signal respectively with a first reference signal that is a first circular function of the third frequency and a second reference signal that is a circular function of the third frequency, and producing respectively a first and a second multiplied signal; PA1 carrying out a predetermined low-pass filtering in response to a supply of the first and the second multiplied signal, and producing respectively a first and second filtered signal; PA1 multiplying the first filtered signal and a third reference signal that is a first circular function of the first frequency, and producing a third multiplied signal; PA1 multiplying the second filtered signal and a fourth reference signal that is a second circular function of the first frequency, and producing a fourth multiplied signal; PA1 multiplying the first filtered signal and the fourth reference signal, and producing a fifth multiplied signal; PA1 multiplying the second filtered signal and the third reference signal, and producing a sixth multiplied signal; PA1 adding the third and the fourth multiplied signal, and producing a first added signal; PA1 adding the fifth and the sixth multiplied signal, and producing a second added signal; PA1 carrying out a predetermined low-pass filtering in response to a supply of the first and the second added signal, and producing respectively a third and fourth filtered signal; and PA1 carrying out amplitude assessment in response to a supply of the third and the fourth filtered signal, and detecting the first and second discrimination signal.
Further, the pilot signal P=f(t) which is supplied through the BPF 1 has underdone the AM-modulation, and this may be expressed by the following equation. EQU f(t)=A{1+k.multidot.cos (.omega..sub.s t+.phi.)}.multidot.cos (.omega..sub.s t+.theta.)
wherein .phi. represents a modulated signal for the stereo discrimination signal that has been received, and .theta. represents a phase difference between the pilot signal carrier and the reference signals g.sub.1(t) -g.sub.4(t).
The pilot signal f(t) is multiplied with the reference signals g.sub.1(t) -g.sub.4(t) respectively at the multipliers 6-9, and the multiplication results P.sub.1 and P.sub.2 respectively of the multipliers 6 and 7 are supplied to the adder 10 and the multiplied results P.sub.3 and P.sub.4 respectively of the multipliers 8 and 9 are supplied to the adder 11. The adders 10 and 11 respectively add the signals P.sub.1 and P.sub.2 and the signals P.sub.3 and P.sub.4 and the added results U.sub.1 and U.sub.2 are supplied respectively to the LPFs 12 and 13. The LPFs 12 and 13 respectively remove high harmonics of the signals U.sub.1 and U.sub.2 and produce signals V.sub.1 and V.sub.2, and these signals V.sub.1 and V.sub.2 are supplied to the amplitude assessment circuit 14. This amplitude assessment circuit 14 assesses the amplitudes of the signals V.sub.1 and V.sub.2 and judges the multiplex sound mode of the broadcast in progress.
The processes explained above may be expressed by the following equations.
First, the operation of the multiplier 6 may be expressed as: ##EQU1##
Similarly, the operations of the multipliers 7-9 may be expressed respectively by the equations (2)-(4). ##EQU2##
The output signals V.sub.1 =v.sub.1 (t) and V.sub.2 =v.sub.2 (t) of the LPFs 12 and 13 may be expressed respectively by the following equations (5) and (6). ##EQU3## wherein T(s) represents a transfer function of the LPFs 12 and 13. The amplitude assessment circuit 14 detects the amplitude term A of the discrimination signal from the signals V.sub.1 and V.sub.2. There are several different approaches for carrying out this detection, but generally this is carried out by squaring each of the signals V.sub.1 and V.sub.2, adding the squared results, and taking 1/2 index of the sum of the squares, hence the square root of the sum of the squares. EQU {v.sub.1 (t).sup.2 +v.sub.2 (t).sup.2 }.sup.1/2 =(k/4)A (7)
If the cut-off frequencies of the LPFs 12 and 13 for the removal of high harmonics are set sufficiently low, it is possible to enhance the equivalent detection characteristics of the discrimination signal. For example, if the cut-off frequency is lowered to about 1 Hz, the frequency 117.5 Hz of the stereo sound discrimination signal may be detected with an accuracy of about .+-.1Hz.
In the conventional pilot signal detection circuit firstly explained above, since the detection sensitivity of the discrimination signal after the AM-detection is largely dependent on the selectivity characteristics of the BPFs of the stereo and dual sound discrimination signals, respectively, it is required that Q of these BPFs be maintained at a high level for purposes of enhancing the selectivity characteristics. This results in limiting the tolerable deviation in the frequency characteristics and leads to problems in that the production yield is lowered and the adaptability to circuit integration is made difficult in the fabrication of circuits.
The conventional pilot signal detection circuit explained secondly above overcomes the problems just mentioned. However, since it is necessary that the cut-off frequency of the LPFs for the removal of the high harmonics after the operation be set to as small as about several Hz for purposes of enhancing the detection accuracy, there is a slow response characteristic in the transmission signal and this results in a longer detection time.
Also, since the pilot signal is supplied directly to the multipliers, it is necessary that, in consideration of the possibility of noise being mixed in during a weak electric field operation, the linearity and the dynamic range of the multipliers be made sufficiently large in order to ensure the accuracy in detection. This adds to the factors of increasing the costs of design and fabrication.